Unconjugated bilirubin inhibits collagen induced platelet activation

Simultaneous determination of free biliverdin and free bilirubin in serum: A comprehensive LC-MS approach

BACKGROUND

Prognosis, diagnosis, and treatment of several diseases strongly rely on the sensitive, selective, and accurate determination of specific biomarkers in relevant biological samples. Free biliverdin and free bilirubin represent important new biomarkers of oxidative stress, however, the lack of suitable analytical methods for their determination has hindered progress in biomedical and clinical research.

RESULTS

Here, we introduce a first comprehensive approach for robust and simultaneous determination of these bilins in serum using liquid chromatography - mass spectrometry (LC-MS). The developed analytical method exhibits linearity for both analytes within the concentration range of 0.5-100 nM, with limits of detection and quantitation determined at 0.1 nM and 0.5 nM, respectively. Moreover, several analytical pitfalls related to the intrinsic molecular structures of free bilirubin and free biliverdin and their trace concentration levels in biological samples are discussed here in detail for the first time. We have shown that the solubility, chemical stability, and affinity of these bilins to various materials strongly depend on the solvent, pH, and addition of stabilizing and chelating agents. Finally, the validated LC-MS method was successfully applied to the analysis of both bilins in fetus bovine serums, yielding higher free bilirubin/biliverdin ratios compared with previously reported values for human serum.

SIGNIFICANCE

Failure to recognize and address the challenges presented here often leads to substantial analytical errors and consequently biased interpretation of the obtained results. This pertains not only to LC-MS, but also to many other analytical platforms due to the compound-derived sources of error.

Establishment of a Predictive Model for Poor Prognosis of Incomplete Revascularization in Patients with Coronary Heart Disease and Multivessel Disease

OBJECTIVE

To establish a predictive model for poor prognosis after incomplete revascularization (ICR) in patients with multivessel coronary artery disease (MVD).

METHODS

Clinical data of 757 patients with MVD and ICR after percutaneous coronary intervention (PCI) in the Affiliated Hospital of Chengde Medical University from January 2020 to August 2021 were retrospectively collected. The least absolute shrinkage and selection operator regression method was used to screen variables, and multivariate logistic regression was used to establish a predictive model. An independent cohort was used to validate the model. The C-statistic was used to verify and evaluate the discriminative ability of the model; the calibration curve was drawn, and the decision curve analysis (DCA) was performed to evaluate the calibration degree, the clinical net benefit, and the practicability of the model.

RESULTS

The predictive factors included female, age, unconjugated bilirubin, uric acid, low-density lipoprotein, hyperglycemia, total occlusion, and severe tortuosity lesion on coronary angiography. The C-statistic of the training and validation sets were 0.628 and 0.745, respectively. The statistical value of the Hosmer-Lemeshow test for the calibration curve of the training and validation sets were 5.27(P = 0.873) and 6.27 (P = 0.792), respectively. DCA showed that the model was clinically applicable when the predicted probability value of major adverse cardiovascular events(MACEs) ranged from 0.07 to 0.68.

CONCLUSIONS

We established a predictive model for poor prognosis after ICR in patients with MVD. The predictive and calibration ability and the clinical net benefit of the predictive model were good, indicating that it can be used as an effective tool for the early prediction of poor prognosis after ICR in patients with MVD.

Pharmaceutical strategies for preventing toxicity and promoting antioxidant and anti-inflammatory actions of bilirubin

Bilirubin (BR) is the final product of haem catabolism. Disruptions along BR metabolic/transport pathways resulting from inherited disorders can increase plasma BR concentration (hyperbilirubinaemia). Unconjugated hyperbilirubinemia may induce BR accumulation in brain, potentially causing irreversible neurological damage, a condition known as BR encephalopathy or kernicterus, to which newborns are especially vulnerable. Numerous pharmaceutical strategies, mostly based on hemoperfusion, have been proposed over the last decades to identify new valid, low-risk alternatives for BR removal from plasma. On the other hand, accumulating evidence indicates that BR produces health benefits due to its potent antioxidant, anti-inflammatory and immunomodulatory action with a significant potential for the treatment of a multitude of diseases. The present manuscript reviews both such aspects of BR pharmacology, gathering literature data on applied pharmaceutical strategies adopted to: (i) reduce the plasma BR concentration for preventing neurotoxicity; (ii) produce a therapeutic effect based on BR efficacy in the treatment of many disorders.

Evaluation of qSOFA score, and conjugated bilirubin and creatinine levels for predicting 28‑day mortality in patients with sepsis

Sepsis is a dangerous disease that develops rapidly and has a high mortality rate. A timely and accurate assessment of the patient's condition is beneficial in improving prognosis and reducing mortality. Therefore, the present study was designed to investigate the potential association between quick sequential organ failure assessment (qSOFA) scores and biochemical indicators, such as conjugated bilirubin (CB) and creatinine levels, with the 28-day prognosis of patients with sepsis in a retrospective observational study. All cases were divided into survival and non-survival groups on the 28th day after diagnosis. The qSOFA scores, and CB and creatinine levels were significantly higher in the non-survival group than in the survival group (both P<0.01). Cox regression models identified CB [hazard ratio (HR), 1.006; P=0.002] and creatinine levels (HR, 1.002; P=0.024) as independent factors affecting 28-day mortality. The area under the curve (AUC) for CB and creatinine levels plus qSOFA score was 0.792 (95% confidence interval, 0.745-0.834), which was larger than the values for CB level, creatinine level and qSOFA score alone (all P<0.01) in the prognosis of 28-day mortality. The cut-off value of CB and creatinine levels plus qSOFA score for the 28-day mortality was 0.275 (-2.466 + 0.012 x CB + 0.002 x creatinine + 1.289 x qSOFA). Patients with lower combined predictor values had a better prognosis as demonstrated by Kaplan-Meier survival curves (log-rank test, 10.060; P=0.002). In both the septic shock and sepsis groups, the combined predictor value was higher in the non-survival group than in the survival group (P<0.001). Therefore, an increase in the combined predictor value of CB and creatinine levels plus qSOFA score may be an important predictor of disease progression and prognosis in patients with sepsis and septic shock.

Bilirubin as a Therapeutic Molecule: Challenges and Opportunities

There is strong evidence that serum free bilirubin concentration has significant effects on morbidity and mortality in the most significant health conditions of our times, including cardiovascular disease, diabetes, and obesity/metabolic syndrome. Supplementation of bilirubin in animal and experimental models has reproduced these protective effects, but several factors have slowed the application bilirubin as a therapeutic agent in human patients. Bilirubin is poorly soluble in water, and is a complex molecule that is difficult to synthesize. Current sources of this molecule are animal-derived, creating concerns regarding the risk of virus or prion transmission. However, recent developments in nanoparticle drug delivery, biosynthetic strategies, and drug synthesis have opened new avenues for applying bilirubin as a pharmaceutical agent. This article reviews the chemistry and physiology of bilirubin, potential clinical applications and summarizes current strategies for safe and efficient drug delivery.

Physiologically increased total bilirubin is associated with reduced risk of first myocardial infarction: A meta-analysis and dose-response analysis

AIM

Bilirubin has potential predictive and prognostic value for myocardial infarction (MI), but the clinical evidence remains controversial. We performed this meta-analysis to systematically quantify the relationships between circulating bilirubin levels and the incidence of MI and post-MI adverse events.

DATA SYNTHESIS

We searched the PubMed, Cochrane Library, Embase, and Web of Science databases for ad-hoc studies, published up to October 17, 2020, recording bilirubin before MI (predictive analyses) or adverse events (prognostic analyses). Relative risks (RR) were pooled by a random-effects model. The dose-response analysis was conducted by restricted cubic splines. In patients without previous MI, increased total bilirubin (TB) reduced the risk of long-term (>3 year) first MI by 22% (95% confidence interval [CI]: 0.69-0.88, n = 4). The dose-response analysis indicated that the RR for first MI decreased by 2.7% per each 2 μmol/L increment of TB (three studies, 95% CI: 1.3%-4.1%, P < 0.001), with a cut-off value of 12.60 μmol/L for RR > 1.00. Elevated bilirubin reduced the incidence of first and recurrent MI by 36% (95% CI: 0.42-0.98, n = 7). However, after suffering MI, higher TB concentrations could not decrease the risk of recurrent MI (RR: 1.02, 95% CI: 0.67-1.55, n = 5) and increased the incidence of short-term (<1 year) post-MI major adverse cardiovascular events, all-cause mortality, and cardiovascular mortality, but not long-term (≥1 year).

CONCLUSION

Higher TB levels within a physiological range reduced the incidence of long-term first MI, with a cut-off value of 12.60 μmol/L.

The role of bilirubin to albumin ratio as a predictor for mortality in critically ill patients without existing liver or biliary tract disease

Background:

Hyperbilirubinemia and hypoalbuminemia are frequently appeared and associated with poor prognosis in critically ill patients. We aim to evaluate the association between the bilirubin to albumin ratio and prognosis in intensive care unit (ICU) patients.

Methods:

This was a retrospective study of 731 patients who were admitted to the medical intensive care unit (MICU) at a tertiary-care center from July 2015 to September 2017. We analyzed the bilirubin to albumin ratio on admission to the MICU, including clinical characteristics and other examinations.

Results:

The overall 28-day survival of MICU patients was 69.1%. On univariate analysis, Acute Physiology and Chronic Health Evaluation (APACHE) II score (P<0.001), Sequential Organ Failure Assessment score (P<0.001), Simplified Acute Physiology Score II score (P<0.001), Creactive protein (P=0.015), and bilirubin/albumin ratio (P<0.001) were associated with mortality of ICU patients. The receiver operating characteristic curves for ICU patients mortality between bilirubin to albumin ratio and APACHE II score were not statistically significant (P=0.282). On multivariate analysis, higher APACHE II score (hazard ratio [HR], 1.05; 95% CI, 1.03 to 1.06; P<0.001) and bilirubin to albumin ratio (HR, 1.65; 95% CI, 1.23 to 2.20; P=0.001) were independently related to the ICU patient mortality.

Conclusions:

A higher bilirubin to albumin ratio was related to the unfavorable prognosis and mortality in critically ill patients.

Heme Oxygenase Dependent Bilirubin Generation in Vascular Cells: A Role in Preventing Endothelial Dysfunction in Local Tissue Microenvironment?

Among antioxidants in the human body, bilirubin has been recognized over the past 20 years to afford protection against different chronic conditions, including inflammation and cardiovascular disease. Moderate increases in plasma concentration and cellular bilirubin generation from metabolism of heme via heme oxygenase (HMOX) in virtually all tissues can modulate endothelial and vascular function and exert antioxidant and anti-inflammatory roles. This review aims to provide an up-to-date and critical overview of the molecular mechanisms by which bilirubin derived from plasma or from HMOX1 activation in vascular cells affects endothelial function. Understanding the molecular actions of bilirubin may critically improve the management not only of key cardiovascular diseases, but also provide insights into a broad spectrum of pathologies driven by endothelial dysfunction. In this context, therapeutic interventions aimed at mildly increasing serum bilirubin as well as bilirubin generated endogenously by endothelial HMOX1 should be considered.

Association Between the Change in Total Bilirubin and Risk of Bleeding Among Patients With Nonvalvular Atrial Fibrillation Taking Dabigatran

There is still a lack of effective biomarkers for the prediction of the risk of bleeding events among patients with nonvalvular atrial fibrillation (NVAF) taking dabigatran. This study aimed to investigate the association between change in total bilirubin (CTBIL) and risk of bleeding among patients with NVAF taking dabigatran. The CTBIL was the difference in serum total bilirubin at out of follow-up from baseline serum total bilirubin. A total of 486 patients with NVAF treated with dabigatran (110 mg twice daily) were recruited from 12 centers in China from February 2015 to December 2017. All patients were followed for 3 months. Cox proportional hazards regression analysis was used to evaluate the association between the CTBIL and bleeding. Moreover, a Cox proportional hazards regression with cubic spline functions and smooth curve fitting (the penalized spline method) and 2 piecewise Cox proportional hazards models were used to address the nonlinearity between CTBIL and bleeding. The mean (SD) follow-up duration was 81.2 (20.2) days. In all, 67 patients experienced bleeding events. A U-shaped association was observed between the CTBIL and bleeding, with increased hazard ratios (HRs) in relation to either low or high CTBIL levels. For CTBIL <6.63 µmol/L, the HR (95% confidence interval [CI]) was 0.90 (0.84-0.96), and for CTBIL ≥6.63 µmol/L, the HR (95% CI) was 1.35 (1.14-1.60). Our findings showed a U-shaped relationship between CTBIL and bleeding. Both low and high levels of CTBIL were associated with a higher risk of bleeding.

The impact of bilirubin ditaurate on platelet quality during storage

Bilirubin ditaurate (BRT), a conjugated bilirubin analogue, has demonstrated anti-platelet characteristics following acute ex vivo exposure. Scavenging of mitochondrial superoxide and attenuation of granule exocytosis suggested a potential benefit for including BRT for storage. With no reports of cytotoxicity following acute exposure, the impact of 35µM BRT on platelet function was investigated, in clinically suppled units, for up to seven days. Exposure to 35µM BRT significantly reduced mitochondrial membrane potential and increased glucose consumption until exhaustion after 72 hours. Platelet aggregation and activation was significantly impaired by BRT. Mitochondrial superoxide production and phosphatidylserine expression were significantly elevated following glucose exhaustion, with decreased viability observed from day five onwards. Lactate accumulation and loss of bicarbonate, support a metabolic disturbance, leading to a decline of quality following BRT inclusion. Although acute ex vivo BRT exposure reported potentially beneficial effects, translation from acute to chronic exposure failed to combat declining platelet function during storage. BRT exposure resulted in perturbations of platelet quality, with the utility of BRT during storage therefore limited. However, these are the first data of prolonged platelet exposure to analogues of conjugated bilirubin and may improve our understanding of platelet function in the context of conjugated hyperbilirubinemia.

Acute bilirubin ditaurate exposure attenuates ex vivo platelet reactive oxygen species production, granule exocytosis and activation

Background

Bilirubin, a by-product of haem catabolism, possesses potent endogenous antioxidant and platelet inhibitory properties. These properties may be useful in inhibiting inappropriate platelet activation and ROS production; for example, during storage for transfusion. Given the hydrophobicity of unconjugated bilirubin (UCB), we investigated the acute platelet inhibitory and ROS scavenging ability of a water-soluble bilirubin analogue, bilirubin ditaurate (BRT) on ex vivo platelet function to ascertain its potential suitability for inclusion during platelet storage.

Methods

The inhibitory potential of BRT (10–100 μM) was assessed using agonist induced platelet aggregation, dense granule exocytosis and flow cytometric analysis of P-selectin and GPIIb/IIIa expression. ROS production was investigated by analysis of H₂DCFDA fluorescence following agonist simulation while mitochondrial ROS production investigated using MitoSOX™ Red. Platelet mitochondrial membrane potential and viability was assessed using TMRE and Zombie Green™ respectively.

Results

Our data shows ≤35 μM BRT significantly inhibits both dense and alpha granule exocytosis as measured by ATP release and P-selectin surface expression, respectively. Significant inhibition of GPIIb/IIIa expression was also reported upon ≤35 μM BRT exposure. Furthermore, platelet exposure to ≤10 μM BRT significantly reduces platelet mitochondrial ROS production. Despite the inhibitory effect of BRT, platelet viability, mitochondrial membrane potential and agonist induced aggregation were not perturbed.

Conclusions

These data indicate, for the first time, that BRT, a water-soluble bilirubin analogue, inhibits platelet activation and reduces platelet ROS production ex vivo and may, therefore, may be of use in preserving platelet function during storage.

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The impact of conjugated bilirubin on platelet function has not been investigated to date.

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Bilirubin ditaurate (BDT) is a water-soluble analogue of conjugated bilirubin.

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BDT attenuates ex vivo platelet activation and ROS generation.

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Conjugated forms of bilirubin might inhibit platelet activation during storage.

Blood platelet function abnormalities in cirrhotic patients with esophageal varices in relation to the variceal bleeding history

Objective: The study aimed at assessing the effect of thrombocytopenia and platelet function abnormalities on the occurrence of variceal bleeding in patients with cirrhosis. Methods: The results of impedance aggregometry, von Willebrand factor antigen level and thromboelastometry (TEM) with and without the addition of a platelet inhibitor (FIBTEM^®, EXTEM^® test, respectively) were compared in two patient groups: Group 1 (n = 32) - patients with moderate or large esophageal or gastric varices, who had never had symptoms of acute gastrointestinal bleeding and Group 2 (n = 26) - patients with history of variceal bleeding. Results: Standard clotting test indicated more hypocoagulable profile in Group 2 compared to Group 1. However, no differences in any TEM component were observed between groups in EXTEM^® test. The contribution of platelets to clot strength was significantly higher in Group 2 than in Group 1 [PLT% = 74.2 (67.5-80.4) versus 68.8 (63.7-76.5) %; p = .039]. The aggregation index was also higher in Group 2 compared to Group 1, although not statistically significant [% of healthy = 96.9 (73.2-140.1) versus 67.6 (52.5-118.8) %, p = .195]. No differences in vWF antigen levels were observed between groups. Conclusions: The results of thromboelastometry and aggregometry indicate increased contribution of platelets in clot formation in patients with a history of variceal bleeding compared to cirrhotic patients who never bled. Comparable effectiveness of hemostasis in both groups is most likely associated with the compensatory role of platelets. Increased platelet activity in this group of patients is probably due to a mechanism independent of the von Willebrand factor antigen level.

Bilirubin acts as a multipotent guardian of cardiovascular integrity: more than just a radical idea

Bilirubin, a potentially toxic catabolite of heme and indicator of hepatobiliary insufficiency, exhibits potent cardiac and vascular protective properties. Individuals with Gilbert's syndrome (GS) may experience hyperbilirubinemia in response to stressors including reduced hepatic bilirubin excretion/increased red blood cell breakdown, with individuals usually informed by their clinician that their condition is of little consequence. However, GS appears to protect from all-cause mortality, with progressively elevated total bilirubin associated with protection from ischemic heart and chronic obstructive pulmonary diseases. Bilirubin may protect against these diseases and associated mortality by reducing circulating cholesterol, oxidative lipid/protein modifications, and blood pressure. In addition, bilirubin inhibits platelet activation and protects the heart from ischemia-reperfusion injury. These effects attenuate multiple stages of the atherosclerotic process in addition to protecting the heart during resultant ischemic stress, likely underpinning the profound reduction in cardiovascular mortality in hyperbilirubinemic GS. This review outlines our current knowledge of and uses for bilirubin in clinical medicine and summarizes recent progress in revealing the physiological importance of this poorly understood molecule. We believe that this review will be of significant interest to clinicians, medical researchers, and individuals who have GS.

Increased mortality in critically ill patients with mild or moderate hyperbilirubinemia

PURPOSE

Increased bilirubin concentrations may be related to outcome, but this has not been well studied. We evaluated the relationship between total serum bilirubin levels and outcome in critically ill patients.

MATERIALS AND METHODS

Data were analyzed from adult critically ill patients included in a 1-day international prevalence study after excluding those with an obvious underlying cause of hyperbilirubinemia.

RESULTS

Serum bilirubin concentrations were >1 mg/dL in 2803 (31%) of 8973 patients, and these patients had higher mortality rates than patients with lower bilirubin concentrations (30% vs 21%, P<.01). For serum bilirubin values of 1.1-6 mg/dL, there was a linear increase in crude mortality (R²=0.96), with the highest hospital mortality (42%) in patients with bilirubin concentrations between 3.7 and 6.0 mg/dL. There was no further increase in the mortality rates for patients with bilirubin concentrations >6 mg/dL. A serum bilirubin concentration >1 mg/dL was an independent risk factor for mortality in multilevel analysis.

CONCLUSIONS

Hyperbilirubinemia without a recognized cause was common and independently associated with increased mortality. There was a linear correlation of mortality with bilirubin concentration for values between 1 and 6 mg/dL but not for higher values.

Serum antioxidant levels associated with subcortical ischemic vascular disease

OBJECTIVE

We aimed to examine changes in serum bilirubin and uric acid (ua) levels in subcortical ischemic vascular disease (Sivd). in addition, we investigated if altered serum bilirubin and ua levels correlate with the subtypes of Sivd as well as the severity of leukoaraiosis (la).

METHODS

this cross-sectional study included 1098 consecutive patients with slight symptoms, such as dizziness, vertigo etc. according to magnetic resonance imaging (Mri) appearances, they were divided into either Sivd group or controls (Cn), and the Sivd group was further grouped in lacunar infarction (li) and la subtypes, as well as different grades. Serum bilirubin and ua levels were determined by the vanadate oxidase method and enzymatic method respectively, after at least an eight hour overnight fasting, in all subjects.

RESULTS

the bilirubin level was obviously lower while the ua level was significantly higher in the Sivd group when compared with the controls. Moreover, the la subgroup presented more significant changes in bilirubin and ua when compared to the li subgroup in both males and females. the correlation was positive between the ua levels and the la severity (r=0.134, p=0.006). Multivariate regression analysis revealed that the odds ratio (95% Ci) for Sivd in the lowest tertile of total bilirubin (tbil<9.58 μmol/l) and highest tertile of ua (ua>339 μmol/l) were 2.702(1.936-3.770) and 2.135(1.521-2.996) respectively after adjusting for confounding variables.

CONCLUSION

Serum bilirubin levels were lower, whereas ua levels were higher in Sivd patients when compared with controls in both males and females, especially in la patients. Moreover, serum ua levels positively correlated to la severity.

Serum bilirubin levels are inversely associated with PAI-1 and fibrinogen in Korean subjects

OBJECTIVES

Oxidative stress may contribute to atherosclerosis and increased activation of the coagulation pathway. Bilirubin may reduce activation of the hemostatic system to inhibit oxidative stress, which would explain its cardioprotective properties shown in many epidemiological studies. This study investigated the association of serum bilirubin with fibrinogen and plasminogen activator inhibitor-1 (PAI-1), respectively.

METHODS

A cross-sectional analysis was performed on 968 subjects (mean age, 56.0 ± 11.2 years; 61.1% men) undergoing a general health checkup. Serum biochemistry was analyzed including bilirubin subtypes, insulin resistance (using homeostasis model of assessment [HOMA]), C-reactive protein (CRP), fibrinogen, and PAI-1.

RESULTS

Compared with subjects with a total bilirubin (TB) concentration of <10.0 μmol/L, those with a TB concentration of >17.1 μmol/L had a smaller waist circumference, a lower triglyceride level, a lower prevalence of metabolic syndrome, and decreased HOMA-IR and CRP levels. Correlation analysis revealed linear relationships of fibrinogen with TB and direct bilirubin (DB), whereas PAI-1 was correlated with DB. After adjustment for confounding factors, bilirubin levels were inversely associated with fibrinogen and PAI-1 levels, respectively. Multivariate regression models showed a negative linear relationship between all types of bilirubin and fibrinogen, whereas there was a significant linear relationship between PAI-1 and DB.

CONCLUSIONS

High bilirubin concentrations were independently associated with low levels of fibrinogen and PAI-1, respectively. The association between TB and PAI-1 was confined to the highest TB concentration category whereas DB showed a linear association with PAI-1. Bilirubin may protect against the development of atherothrombosis by reducing the hemostatic response.

Unconjugated Bilirubin exerts Pro-Apoptotic Effect on Platelets via p38-MAPK activation

Thrombocytopenia is one of the most frequently observed secondary complications in many pathological conditions including liver diseases, where hyperbilirubinemia is very common. The present study sought to find the cause of thrombocytopenia in unconjugated hyperbilirubinemic conditions. Unconjugated bilirubin (UCB), an end-product of heme catabolism, is known to have pro-oxidative and cytotoxic effects at high serum concentration. We investigated the molecular mechanism underlying the pro-apoptotic effect of UCB on human platelets in vitro, and followed it up with studies in phenylhydrazine-induced hyperbilirubinemic rat model and hyperbilirubinemic human subjects. UCB is indeed found to significantly induce platelet apoptotic events including elevated endogenous reactive oxygen species generation, mitochondrial membrane depolarization, increased intracellular calcium levels, cardiolipin peroxidation and phosphatidylserine externalization (p < 0.001) as evident by FACS analysis. The immunoblots show the elevated levels of cytosolic cytochrome c and caspase activation in UCB-treated platelets. Further, UCB is found to induce mitochondrial ROS generation leading to p38 activation, followed by downstream activation of p53, ultimately resulting in altered expression of Bcl-2 and Bax proteins as evident from immunoblotting. All these parameters conclude that elevated unconjugated bilirubin causes thrombocytopenia by stimulating platelet apoptosis via mitochondrial ROS-induced p38 and p53 activation.

Looking to the horizon: the role of bilirubin in the development and prevention of age-related chronic diseases

Bilirubin, the principal tetrapyrrole, bile pigment and catabolite of haem, is an emerging biomarker of disease resistance, which may be related to several recently documented biological functions. Initially believed to be toxic in infants, the perception of bilirubin has undergone a transformation: it is now considered to be a molecule that may promote health in adults. Data from the last decade demonstrate that mildly elevated serum bilirubin levels are strongly associated with reduced prevalence of chronic diseases, particularly cardiovascular diseases (CVDs), as well as CVD-related mortality and risk factors. Recent data also link bilirubin to other chronic diseases, including cancer and Type 2 diabetes mellitus, and to all-cause mortality. Therefore, there is evidence to suggest that bilirubin is a biomarker for reduced chronic disease prevalence and a predictor of all-cause mortality, which is of important clinical significance. In the present review, detailed information on the association between bilirubin and all-cause mortality, as well as the pathological conditions of CVD, cancer, diabetes and neurodegenerative diseases, is provided. The mechanistic background concerning how bilirubin and its metabolism may influence disease prevention and its clinical relevance is also discussed. Given that the search for novel biomarkers of these diseases, as well as for novel therapeutic modalities, is a key research objective for the near future, bilirubin represents a promising candidate, meeting the criteria of a biomarker, and should be considered more carefully in clinical practice as a molecule that might provide insights into disease resistance. Clearly, however, greater molecular insight is warranted to support and strengthen the conclusion that bilirubin can prevent disease, with future research directions also proposed.

Low-normal thyroid function and novel cardiometabolic biomarkers

The concept is emerging that low-normal thyroid function, i.e., either higher thyroid-stimulating hormone or lower free thyroxine levels within the euthyroid reference range, could contribute to the development of atherosclerotic cardiovascular disease. It is possible that adverse effects of low-normal thyroid function on cardiovascular outcome may be particularly relevant for specific populations, such as younger people and subjects with high cardiovascular risk. Low-normal thyroid function probably relates to modest increases in plasma total cholesterol, low density lipoprotein cholesterol, triglycerides and insulin resistance, but effects on high density lipoprotein (HDL) cholesterol and non-alcoholic fatty liver disease are inconsistent. Low-normal thyroid function may enhance plasma cholesteryl ester transfer, and contribute to an impaired ability of HDL to inhibit oxidative modification of LDL, reflecting pro-atherogenic alterations in lipoprotein metabolism and HDL function, respectively. Low-normal thyroid function also confers lower levels of bilirubin, a strong natural anti-oxidant. Remarkably, all these effects of low-normal thyroid functional status appear to be more outspoken in the context of chronic hyperglycemia and/or insulin resistance. Collectively, these data support the concept that low-normal thyroid function may adversely affect several processes which conceivably contribute to the pathogenesis of atherosclerotic cardiovascular disease, beyond effects on conventional lipoprotein measures.

Bilirubin, platelet activation and heart disease: a missing link to cardiovascular protection in Gilbert's syndrome?

Gilbert's syndrome (GS) is a relatively common condition, inducing a benign, non-hemolytic, unconjugated hyperbilirubinemia. Gilbert's Syndrome is associated with mutation in the Uridine Glucuronosyl Transferase 1A1 (UGT1A1) gene promoter, reducing UGT1A1 activity, which normally conjugates bilirubin allowing its elimination from the blood. Individuals with GS demonstrate mildly elevated plasma antioxidant capacity caused by elevated levels of unconjugated bilirubin (UCB), reduced thiols and glutathione. Interestingly, the development of, and risk of mortality from, cardiovascular disease is remarkably reduced in GS individuals. An explanation for this protection may be explained by bilirubin's ability to inhibit multiple processes that induce platelet hyper-reactivity and thrombosis, thus far under-appreciated in the literature. Reactive oxygen species are produced continuously via metabolic processes and have the potential to oxidatively modify proteins and lipids within cell membranes, which may encourage the development of thrombosis and CVDs. Oxidative stress induced platelet hyper-reactivity significantly increases the risk of thrombosis, which can potentially lead to tissue infarction. Here, we discuss the possible mechanisms by which increased antioxidant status might influence platelet function and link this to cardiovascular protection in GS. In summary, this is the first article to discuss the possible role of bilirubin as an anti-thrombotic agent, which inhibits platelet activation and potentially, organ infarction, which could contribute to the reduced mortality rate in mildly hyperbilirbinemic individuals.

Potential cardiovascular risk protection of bilirubin in end-stage renal disease patients under hemodialysis

We evaluated the potential cardiovascular risk protection of bilirubin in hemodialysis (HD) patients. An enlarged set of studies were evaluated in 191 HD patients, including hematological study, lipid profile, iron metabolism, nutritional, inflammatory markers, and dialysis adequacy. The TA duplication screening in the UDP-glucuronosyltransferase 1 A1 (UGT1A1) promoter region was also performed. The UGT1A1 genotype frequencies in HD patients were 49.2%, 42.4%, and 8.4% for 6/6, 6/7, and 7/7 genotypes, respectively. Although no difference was found in UGT1A1 genotype distribution between the three tertiles of bilirubin, significant differences were found with increasing bilirubin levels, namely, a decrease in platelet, leukocyte, and lymphocyte counts, transferrin, oxidized low-density lipoprotein (ox-LDL), ox-LDL/low-density lipoprotein cholesterol ratio, apolipoprotein (Apo) A, Apo B, and interleukin-6 serum levels and a significant increased concentration of hemoglobin, hematocrit, erythrocyte count, iron, transferrin saturation, Apo A/Apo B ratio, adiponectin, and paraoxonase 1 serum levels. After adjustment for age these results remained significant. Our data suggest that higher bilirubin levels are associated with beneficial effects in HD patients, by improving lipid profile and reducing the inflammatory grade, which might contribute to increase in iron availability. These results suggest a potential cardiovascular risk protection of bilirubin in HD patients.

Circulating bilirubin and defense against kidney disease and cardiovascular mortality: mechanisms contributing to protection in clinical investigations

Unconjugated bilirubin is an endogenous circulating antioxidant, bound to albumin, and therefore is retained in the vascular compartment. Bilirubin has well-documented neurotoxic effects in infants; however, current evidence indicates mildly elevated bilirubin is associated with protection from cardiovascular disease and all-cause mortality in adults. Recent clinical studies show mildly elevated bilirubin is associated with protection from kidney damage and dysfunction, in addition to cardiovascular events and all-cause mortality in patients undergoing hemodialysis. This is the first review to examine the clinical evidence and summarize the potential mechanisms of action that link bilirubin to protection from kidney damage, subsequent kidney failure, and dialysis-related mortality. With this understanding, it is hoped that new therapies will be developed to prevent renal dysfunction and mortality from cardiovascular disease in at-risk individuals.

Association of bilirubin with coronary artery calcification and cardiovascular events in the general population without known liver disease: the Heinz Nixdorf Recall study

BACKGROUND

Bilirubin is a product of heme oxygenase and an antioxidant per se. In selected cohorts and patient studies, bilirubin was associated with decreased risk for cardiovascular events. We aimed to determine the association of serum bilirubin with coronary artery calcification (CAC) and incident cardiovascular disease in the unselected general population.

METHODS

Participants without prior coronary heart or liver disease were drawn from the population-based Heinz Nixdorf Recall study. CAC was measured from electron beam computed tomography and quantified using the Agatston method. Incident major cardiovascular events (coronary events, stroke, cardiovascular death) were assessed during follow-up. The association of bilirubin with CAC score and incident events was calculated using regression analysis.

RESULTS

A total of 3,553 subjects (mean age 59.4 years, 44 % male) were included. Bilirubin was inversely correlated with CAC in men [estimated % change in (CAC + 1) per each gender-specific standard deviation of bilirubin [95 % confidence interval (CI): -6.1 (-11.6; -1.7) %, p = 0.032] but less so in women [-3.6 (-7.7; 0.2) %, p = 0.065], when adjusting for age only. With adjustment for traditional cardiovascular risk factors, the association was less pronounced [men: -2.5 (-7.5; 2.6) %, p = 0.35, women: -0.2 (-4.1; 3.5) %, p = 0.9]. Likewise, higher bilirubin by one standard deviation was associated with 19 % lower frequency of incident cardiovascular events in age- and gender-adjusted Cox regression analysis [hazard ratio (95 % CI): 0.81 (0.68; 0.96), p = 0.014]; this effect was slightly attenuated after adjustment for traditional cardiovascular risk factors [0.87 (0.73; 1.04), p = 0.13].

CONCLUSION

Our data from a general population suggest that a potential protective role of bilirubin in the atherosclerosis process is largely secondary to a more beneficial risk factor profile.

A review of the mechanisms and effectiveness of dietary polyphenols in reducing oxidative stress and thrombotic risk

Dietary sources of polyphenols, which are derivatives and/or isomers of flavones, isoflavones, flavonols, catechins and phenolic acids, possess antioxidant properties and therefore might be important in preventing oxidative-stress-induced platelet activation and attenuating adverse haemostatic function. Free radicals, including reactive oxygen and nitrogen species, promote oxidative stress, leading to platelet hyperactivation and the risk of thrombosis. The consumption of antioxidant/polyphenol rich foods might therefore impart anti-thrombotic and cardiovascular protective effects via their inhibition of platelet hyperactivation or aggregation. Most commonly-used anti-platelet drugs such as aspirin block the cyclooxygenase (COX)-1 pathway of platelet activation, similar to the action of antioxidants with respect to neutralising hydrogen peroxide (H2 O2 ), with a similar effect on thromboxane production via the COX-1 pathway. Polyphenols also target various additional platelet activation pathways (e.g. by blocking platelet-ADP, collagen receptors); thus alleviating fibrinogen binding to platelet surface (GPIIb-IIIa) receptors, reducing further platelet recruitment for aggregation and inhibiting platelet degranulation. As a result of the ability of polyphenols to target additional pathways of platelet activation, they may have the potential to substitute or complement currently used anti-platelet drugs in sedentary, obese, pre-diabetic or diabetic populations who can be resistant or sensitive to pharmacological anti-platelet therapy.

Bilirubin and related tetrapyrroles inhibit food-borne mutagenesis: a mechanism for antigenotoxic action against a model epoxide

Bilirubin exhibits antioxidant and antimutagenic effects in vitro. Additional tetrapyrroles that are naturally abundant were tested for antigenotoxicity in Salmonella. Un-/conjugated bilirubin (1 and 2), biliverdin (4), bilirubin and biliverdin dimethyl esters (3 and 5), stercobilin (6), urobilin (7), and protoporphyrin (8) were evaluated at physiological concentrations (0.01-2 μmol/plate; 3.5-714 μM) against the metabolically activated food-borne mutagens aflatoxin B1 (9) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (10). Compound 8 most effectively inhibited the mutagenic effects of 9 in strain TA102 and 10 in TA98. Compound 7 inhibited 9-induced mutagenesis in strain TA98 most effectively, while 1 and 4 were promutagenic in this strain. This is likely due to their competition with mutagens for phase-II detoxification. Mechanistic investigations into antimutagenesis demonstrate that tetrapyrroles react efficiently with a model epoxide of 9, styrene epoxide (11), to form covalent adducts. This reaction is significantly faster than that of 11 with guanine. Hence, the evaluated tetrapyrroles inhibited genotoxicity induced by poly-/heterocyclic amines found in foods, and novel evidence obtained in the present investigation suggests this may occur via chemical scavenging of genotoxic metabolites of the mutagens investigated. This may have important ramifications for maintaining health, especially with regard to cancer prevention.